Photoconductivity spectra of polymeric devices using poli(p-phenylene vinylene) (PPV) derivatives as the active layer

Spectral photoconductivity measurements were carried out in ITO/Polymer/Metal structures using three different polymers as the active layer: poly(2-methoxy-5-(2'-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV), poly(2-methoxy-5-decyloxy-pphenylene vinylene) (OC10-PPV) and a phenyl-substituted PPV copolymer. In order to observe the influence of the work-function of the cathode on the photoconductivity spectra, different metallic electrodes (Cu, Al and Mg) were deposited on the top of the device structure. The optical properties of the polymeric films were analyzed by means of absorption and stationary fluorescence spectra. The photoconductive behavior of the devices was investigated from the current-voltage (I-V) curves under different illumination wavelengths and from the steady-state photocurrent spectra in the dc regime. The devices were illuminated either through the ITO an orde or the metallic cathode to verify the influence of the penetration profile of light on the photoconductivity. The photocurrent spectral data under different experimental conditions were adjusted by using a model developed taking into account both the intrinsic and the extrinsic components of the photoconductivity. From this model, it was possible to obtain relevant information about the polymers used as the active layers, such as the nature of the major recombination mechanism and the internal efficiency of carrier photogeneration in the bulk and at the device interfaces.